National and international standards define the manner in which electric circuits of LV installations must be realized, and the capabilities and limitations of the various switching devices which are collectively referred to as switchgear.

The main functions of switchgear are:

Electrical protection

Electrical isolation of sections of an installation

Local or remote switching

These functions are summarized below in Figure H1.

Electrical protection at low voltage is (apart from fuses) normally incorporated in circuit-breakers, in the form of thermal-magnetic devices and/or residual-currentoperated tripping devices (less-commonly, residual voltage- operated devices - acceptable to, but not recommended by IEC).

In addition to those functions shown in Figure H1, other functions, namely:

Over-voltage protection

Under-voltage protection

are provided by specific devices (lightning and various other types of voltage-surge arrester, relays associated with contactors, remotely controlled circuit-breakers, and with combined circuit-breaker/isolators… and so on)